Conventionally, for example, in an electrical power steering apparatus, the apparatus comprises an electrical motor for imparting steering assist torque in accordance with the turning operation of the steering wheel, and controls the power supply for the electrical motor to adjust the steering assist torque. Such an electrical power steering apparatus uses an in-vehicle battery, and consumes large electrical power. Therefore, for example, the apparatus proposed in Japanese Patent Application Laid-Open (kokai) No. 2007-91122 comprises a subsidiary power supply for aiding an in-vehicle battery. The subsidiary power supply has a configuration where the subsidiary power supply can be connected in parallel with an electrical power supply line from the in-vehicle battery (hereinafter, will be referred to as a “main power supply”) to a motor drive circuit, be charged by the main power supply, and supply electrical power to the motor drive circuit by using its stored electrical power. In addition, this apparatus comprises a voltage converter, and converts the output voltage of the main power supply to supply electrical power to the motor drive circuit.